Will induction motors get replaced by synchronous motors?

[bonzer]

Hello everyone! Please give me your opinion about it. Will induction motors get replaced by synchronous motors? I'm attending a course about motors at university but we study synchronous motors only. At the same time I heard that induction motors are very popular even though less efficient. I'm afraid that anyway nobody's gonna replace them in near future in industrial environment, which is the domain where I'm gonna work and it feels like they aren't that popular in this case. That's why I hope I could understand them by my own...

[rstofer]

Efficiency of induction motors has increased considerably over the last 40 years to the point that there is no financial incentive to change them to something else.  Coupled with a variable frequency drive, they can operate in unison with the demands imposed by the load.  This is a really big deal for HVAC heating/cooling water pumps and air handlers.  The MEs will always specified larger pumps and fans, the smart EE can recover from that.

Synchronous motors are used in industrial applications and, in particular, for large fixed loads like air moving fans or very large air compressors.  When used in these applications, the motor is often selected to improve plant power factor.  It is easier to control the field excitation if the load is nearly constant.  But I don't think you're going to see synchronous motors used for all applications throughout the plant.

Unless plant power factor is horrible, it is probably not financially possible to justify power factor correction with capacitors or synchronous motors.  Suppose your electric bill is, say, $8M/year and, of that, $20k is power factor penalty.  You would be far better off trying to optimize consumption rather than messing around trying to recover that penalty.  If you think you can fix things by 10%, well, 10% of $20k is $2k while 10% of $8M is $800k.  There's much more savings in reducing consumption.  There's also a bigger raise for having done so.  And, by the way, it's easier!

[SeanB]

99% of electric motors in service are induction motors, the rest are a blend of universal motors, with synchronous motors only taking a tiny portion of that. Only place they are used almost exclusively is in mains synchronous clocks and microwave oven turntables, but even there they are being superseded by other types of motor. Only in the biggest older plants will you still find them, used for large loads and constant loads, where they find retrofitting a VFD too expensive for the age of the plant. Otherwise induction pretty much rules the roost, as you can get all of the advantages of a synchronous motor with an induction motor and an inverter of sorts, including energy recovery as a bonus during stopping, if you use a common DC bus for multiple inverters.

[Zero999]

By synchronous motors are you talking about those with rare earth magnets, rather than those which require excitation? Yes, they're more efficient than induction motors, but are also a heck of a lot more expensive. Modern induction motors with copper, rather than aluminium bars in the rotor are almost efficient as rare earth magnet motors and I believe there has been some investigation into using them in electric vehicles.

Power factor is not an issue because large variable frequency drives include active power factor correction.

[Kleinstein]

Induction motors are a lot easier to start than synchronous motors, that tend to need a special variable frequency drive at least for starting. Synchronous motors have advantages, like slightly higher possible efficiency but they also have disadvantages in many cases and they tend to be more expensive.  Synchronous motors / brushless motor may get attractive if there is speed control anyway.

[Circlotron]

Most so-called brushless DC motors are actually permanent magnet synchronous motors. Some domestic appliances like washing machines nowadays have them. Also inverter driven air conditioners and a very few refrigerators. These are relatively small motors though, not industrial strength ones. Induction motors have the simplicity of connecting directly to the mains if constant speed is all you need. They are not going away any time soon.

[Dave]

There is one major advantage of induction motors over permanent magnet synchronous motors: easy to obtain materials. All you need is copper, aluminium, steel and some insulation.

Rare earth metals are predominantly produced by China. Any export/import restrictions which might be imposed in the near future could have serious impact on the production of these motors.

In terms of electric vehicle propulsion, PMSM is quite favorable at low speeds, as they are very efficient.
The story changes, however, when the speeds are high (cruising on the motorway, for example): a lot of current is needed for field-weakening.

[bonzer]

Thanks to everyone for the answers!
When I said about synchronous motors I didn't mean only permanent magnet motors. There are also synchronous reluctance motors. What do you think about them?  I don't know if these are expensive too but they have low rotor losses compared to induction motors. They also offer a good speed control but their problem is a high torque ripple.

[Circlotron]

Why is your university so out of touch with what is used in the real world?

[T3sl4co1l]

AFAIK, reluctance motors are always bulkier.  There is no magnetic "insulator", only air gap, so the flux density between poles -- in the air gap -- can only be some amount less.  Or, I suppose you could use superconductors to shield it, but, y'know, superconductors.

Steppers are quite common and reasonably powerful, but limited to precision applications, and are still bulkier than their salient-pole friends.  Servo motors also work well here, with higher speed and torque performance at the cost of a more complicated control and feedback system.

Tim

[bonzer]

I don't know exactly why they don't care about them, maybe because there's not enought time. But the professor himself said that industry now is full of asynchronous motors so if you want we can have a lecture about them but we don't include it in the program because it's a matter of time when they are gonna be replaced with what we study.

But from the driving point of view like the power electronics - is there a big difference from working with an asynchronous motor? Is it more difficult to control them? If I tried to understand them by my own? (After all synchronous motor knowledge)

[hermitengineer]

Did they already drop classical computing too, because quantum computing is the wave of the future?

Unless they see strong indicators that these reluctance motors are poised to drive induction motors into extinction before you graduate, that's a very impractical, elitist attitude they're taking.

[atmfjstc]

But the professor himself said that industry now is full of asynchronous motors so if you want we can have a lecture about them but we don't include it in the program because it's a matter of time when they are gonna be replaced with what we study.

University professors have a habit of being excessively liberal and "progressive", but this just takes the cake. So instead of preparing you for reality, the professor is preparing you for what he has unilaterally decided is the future, against all evidence and common sense....

[T3sl4co1l]

An induction motor, you drive flux into the stator, which induces a current in the rotor.  The induced current in turn reflects a magnetic field, which serves as the permanent magnet in a PMAC machine.

The key observations are:
1. The reflected field is proportional to applied field.

The most direct consequence of this is, because flux is limited by stator cross section and flux density, the induced current is small at low frequencies.  In other words, a VFD must reduce voltage in proportion to frequency, and the available torque (before stall) is proportional as well.  So power goes as freq^2, and induction motors are not very useful at low RPM.

This does mean they are very useful above rated speed, given that core losses and ultimate RPM limits are respected, of course!

A 1kW size motor isn't going to last very long driven at say 400Hz+... but will be an impressive show when it grenades.   More modest frequencies (say 25-100Hz) are the useful variable range.


2. The field is not permanent.  It is trapped in a conductor, but a nonideal one, so it slips.

After all, if you had a superconducting rotor, it would be a synchronous machine (although a slightly odd one where the reflected field is proportional to applied field).  Or possibly it wouldn't work because the induced current needs to lag behind the applied field, but I don't think so?  (I'd have to refresh my knowledge of both motors and superconductors to verify that.)

Of course, if we charge the superconductor with a current (and thus field), it will be permanent, and will act exactly like a PM rotor.  (Superconductors exclude external currents, so you can't quite do this in a single step, from the stator field.  It would be an ordinary electromagnet rotor, synchronous machine, that happens to use a superconductor.)

Anyway, the rotor is synchronous with the stator's magnetic field as usual, but because the rotor's own field is not trapped -- it rotates within the rotor itself -- the total RPM is a bit less than the applied mains.  This is why induction motors are labeled 1425 RPM or thereabouts (1750 RPM in US).  The difference is called slip, and is the rate the field rotates in the rotor.  Yep, the eddy currents flowing in the rotor, rotate quite slowly indeed -- this corresponds to an L/R time constant (of the rotor) of a few Hz, not bad, eh?

You can also demonstrate this at ~0 RPM, by biasing the stator with some DC current, and turning the rotor.  This is a highly effective magnetic brake, and this method of operation is sometimes used to stop machinery.

Indeed, the rate of slip is proportional to applied torque, so this could also be used to some extent to gauge load factor; it also means that an induction machine can be used perfectly symmetrically as a generator, you just need to push it faster than the synchronous speed.

You can use an induction machine as a generator (alternator), after supplying some seed current.  Usually there's enough residual magnetism in the rotor steel to get things going, and a capacitor is used (I think) to maintain current between cycles.  Starting a motor from no current, the generated voltage increases exponentially until limited by saturation (which will be close to nominal output voltage, because the stator is designed to run just shy of saturation), and then it's about as good as a synchronous machine; give or take the waveform, which may be ratty due to the saturation, and of course having poor frequency stability because of slip.

Tim

[bonzer]

Tim thanks a lot for your explanation.

Yes, I do agree with you talking about university system and these feelings have always followed me during the years. Sometimes the academic world feels a bit parallel to the reality and not part of it. And I'm sure it's not only here in my university. You can find some papers from different parts of the world writing on google something like "reluctance synchronous motor vs induction motor" . Being cheaper than permanent magnet version, they consider them to be able to replace asynchronous motors in future but as I said the real world is different and complex, we can't be certain about what's gonna happen, I think only if there's a huge cost difference it usually matters.

Whatever, I will try to understand them when I'll have some free time from the internet and by studying the details of what Tim said.

[amyk]

Modern induction motors with copper, rather than aluminium bars in the rotor are almost efficient as rare earth magnet motors and I believe there has been some investigation into using them in electric vehicles.

Interestingly enough, early induction motors used copper too, but switched because aluminium was cheaper...

Personally, I like the sound of a repulsion-start induction motor:

[Dave]

But from the driving point of view like the power electronics - is there a big difference from working with an asynchronous motor? Is it more difficult to control them? If I tried to understand them by my own? (After all synchronous motor knowledge)

In terms of field-oriented control (FOC; controlling the torque of the motor - required in electric vehicle applications, for example), yes, it is quite a bit more involved.

The motor needs to be fairly well characterized (mutual inductance, leakage inductance, stator and rotor resistance, etc.) in order to control it well. The shift of resistance with respect to temperature also needs to be taken into account (copper and aluminium conveniently have very similar temperature coefficients - 4.04e-3 vs 3.90e-3 K^-1, respectively).

On the bright side, they only require incremental encoders. You need to know the exact position of a rotor for FOC of a PMSM, but you only need to know the speed of the rotor with an induction motor. Needless to say, incremental encoders are significantly cheaper than absolute ones.

[Ground_Loop]

I don't know exactly why they don't care about them, maybe because there's not enought time. But the professor himself said that industry now is full of asynchronous motors so if you want we can have a lecture about them but we don't include it in the program because it's a matter of time when they are gonna be replaced with what we study.

But from the driving point of view like the power electronics - is there a big difference from working with an asynchronous motor? Is it more difficult to control them? If I tried to understand them by my own? (After all synchronous motor knowledge)

Synchronous motors are generally started in a two-step process.  Initially, the rotor winding are shorted such that the rotor can behave as a standard induction motor.  Once the rotor speed has reached a certain point the rotor winding short is removed and a DC field current is applied to fix the field orientation relative to the rotor.  At this point the rotor can "lock in" to the armature field rotation and behave in synchronous mode.  So a synchronous motor requires a control mechanism to sense rotor speed and switch the rotor field current.